1. Field of the Invention
The present invention relates to a luminescent screen comprising a resonant microcavity having a phosphor active region.
2. Description of the Prior Art
Conventional cathode ray tube (CRT) displays use electrons emitted from an electron gun and accelerate them through an intense electric field projecting them onto a screen coated with a phosphor material in the form of a powder. The high-energy electrons excite luminescence centers in the phosphors which emit visible light uniformly in all directions. CRT""s are well established in the prior art and are commonly found in television picture tubes, computer monitors and many other devices.
Displays using powder phosphors suffer from several significant limitations, including: low directional luminosity (i.e., brightness in one direction) relative to the power consumed; poor heat transfer and dissipation characteristics; and a limited selection of phosphor chromaticities (i.e., the colors of the light emanating from the excited phosphors).
The directional luminosity is an important feature of a display because the directional properties influence the efficiency with which it can be effectively coupled to other devices (e.g., lenses for projection CRT""s). The normal light flux pattern observed from a luminescent screen closely follows a xe2x80x9cLambertian distributionxe2x80x9d; i.e., light is emitted uniformly in all directions. For direct viewing purposes this is desirable, as the picture can be seen from all viewing angles. However, for certain applications a Lambertian distribution of the light flux is inefficient. These applications include projection displays and the transferring of images to detectors for subsequent image processing.
Heat transfer and dissipation characteristics are important because one of the limiting factors in obtaining bright CRT""s suitable for large screen projection is the heating of the phosphor screen. As the incident electron beam density increases, the phosphor temperature increases. When the phosphor reaches a certain temperature, its luminosity decreases. This is known as thermal quenching. With conventional powder-phosphor displays the phosphor-to-screen heat transfer characteristics are relatively poor, therefore heat dissipation is limited and thermal quenching can occur at relatively low electron beam densities. Because projection displays require high electron beam densities to produce the brightness required to project an image, this inefficiency makes conventional CRT""s poorly suited for projection displays.
Chromaticity is important because the faithful reproduction of colors in a display requires that the three primary-color phosphors (red, green and blue) conform to industry chromaticity standards (e.g., European Broadcasting Union specifications). Finding phosphors for each of the three primary colors that exactly match these specifications is one of the most troublesome aspects of phosphor development.
The decay time of the activator (i.e., light emitting ion in the phosphor) is also another important parameter for a phosphor. In an ideal phosphor for high brightness applications, it is desirable to control directly the decay time of the phosphor for each display application. For example, in some applications, shorter decay times allow rapid re-excitation of the activator with a corresponding increase in the maximum light output. The decay time is typically determined by the natural spontaneous transition rate of the activator. In order to improve phosphor performance it is therefore desirable to have control over this spontaneous transition rate.
Another problem encountered in conventional phosphor displays is that energy can transfer from one activator to another nearby activator in the phosphor host matrix. This is a nonradiative process where the efficiency of the phosphor is reduced. The energy transfer increases with increasing activator concentration and therefore it limits the density of activators that can be incorporated in a display and thus the maximum light output.
The use of a single-crystal, thin-film phosphor as a faceplate for a CRT was first described in a British patent application by M. W. Van Tol, et al., UK Pat. GB-2000173A (1980). This patent taught the use of an yttrium aluminum garnet Y3Al5O12 (YAG) film grown by liquid phase epitaxy (LPE) on a single-crystal YAG substrate. The YAG film is doped with a rare-earth ion which emits light when excited by electrons. (Doping is the process wherein dopant ions are substituted for host ions in the crystal lattice during crystal growth.) In this device, the thickness of the thin-film layer is from one to six microns and does not bear any relation to the wavelength of the light to be emitted by the display.
This device exhibited several advantages over conventional powder-phosphor displays one such advantage was that heat was transferred from the phosphor more efficiently because of the perfect contact between the phosphor and the screen, and because of the high thermal conductivity of the YAG substrate. The screen could be loaded with a higher beam density without exhibiting thermal quenching and, therefore, could produce more light.
Another advantage of single-crystal phosphor luminescent screens versus powder deposited luminescent screens is concerned with the resolution of a pixel (i.e., light producing spot). For high resolution displays using powder phosphor, the limiting size of a pixelxe2x80x94and hence the resolution of the screenxe2x80x94is determined by the particle size of the phosphor powder. Single-crystal phosphors, on the other hand, are not affected by this since they do not contain discrete particles.
Powder phosphors further reduce resolution due to the light scattering from the surface of the powder. Because of the lack of discrete phosphor particles and the absence of light scattering, thin-film displays have high image resolution, limited only by the spot size of the exciting electron beam. The increasing demand for higher resolution displays makes this a particularly attractive advantage.
Yet another advantage is concerned with producing a vacuum in a CRT. To allow the electron beam to travel between the electron gun and the phosphor screen, a vacuum must be maintained within a CRT. Conventional powder phosphors have a high total surface area and, generally, organic compounds are used in their deposition. Both the high surface area and the presence of residual organic compounds cause problems in holding and maintaining a good vacuum in the CRT. Using thin-film phosphors overcomes both of these effects, as the total external surface area of the tube is controlled by the area of the thin-film (which is much less than the surface area of a powder phosphor display) and, furthermore, there are no residual organic compounds present in thin-film displays to reduce the vacuum in the sealed tube.
The thin-film phosphors of Van Tol, et al., exhibit one prohibiting disadvantage, however, due to the phenomenon of xe2x80x9clight piping.xe2x80x9d Light piping is the trapping of light within the thin-film, rendering it incapable of being emitted from the device. This is caused by the total internal reflection of the light rays generated within the thin-film. Since the index of refraction (n) of most phosphors is around n=2, only those light rays whose incident angles are less than the critical angle, xcex8c (where sin xcex8c=1/n) will be emitted from the front of the thin-film. The critical angle for an n=2 material is around 30xc2x0. Therefore, the fraction of light that escapes from the front of the thin-film is only about 6.7% of the total light. The common design of placing a highly reflective aluminum layer behind the film only doubles the output to about 13% of the light. Moreover, this light is spread in a xe2x80x9cLambertian distributionxe2x80x9d and is not directional. As a result of light piping, the external efficiency (i.e., the percentage of photons escaping from the display relative to all photons created in the display) is less than one-tenth that of powder phosphor displays. Therefore, in spite of the unique advantages offered in terms of thermal properties, resolution, and vacuum maintenance; the development of commercial CRT devices based on thin-films is held back by their poor efficiency due to xe2x80x9clight pipingxe2x80x9d.
Some schemes have been designed to reduce the xe2x80x9clight pipingxe2x80x9d problem. One scheme described by Bongers, et al., U.S. Pat. No. 4,298,820 (1981), uses a thin-film, deposited by LPE, with V-shaped grooves etched into the surface to reflect light out of the thin-film. This approach brought about an improvement in external efficiency of around 1 xc2xd to 2 xc2xd times that of a thin-film display without the V-shaped grooves. Given the previous external efficiency of 13%, this would still only lead to a total external efficiency of around 20% to 30%.
Another scheme, described by Huo and Hou, xe2x80x9cReticulated Single-Crystal Luminescent Screenxe2x80x9d, 133 J. Electrochem. Soc. 1492 (1986), involves etching individual mesa shapes onto the thin-film deposited by LPE. This led to a three times improvement in external efficiency (still rendering only about a 30% external efficiency). Furthermore, since the phosphor layer was no longer smooth, any light rays that were internally reflected could find themselves rescattered to areas far from their point of creation, thus spoiling the resolution of the display.
Microcavity resonators, which can be incorporated in the present invention, have existed for some time and have recently been described by H. Yokoyama, xe2x80x9cPhysics and Device Applications of Optical Microcavitiesxe2x80x9d 256 Science 66 (1992). Microcavities are one example of a general structure that has the unique ability to control the decay rate, the directional characteristics and the frequency characteristics of luminescence centers located within them. The changes in the optical behavior of the luminescence centers involve modification of the fundamental mechanisms of spontaneous and stimulated emission. Physically, such structures as microcavities are optical resonant cavities with dimensions ranging from less than one wavelength of light up to tens of wavelengths. These have been typically formed as one integrated structure using thin-film technology. Microcavities involving planar, as well as hemispherical, reflectors have been constructed for laser applications.
Resonant microcavities with semiconductor active layers, for example silicon or GaAs, have been developed as semiconductor lasers and as light-emitting diodes (LEDs).
E. F. Schubert, et al., xe2x80x9cGiant Enhancement of Luminescence Intensity in Er-doped Si/SiO2 Resonant Microcavitiesxe2x80x9d 61(12) Appl. Phys. Lett. 1381 (1992), describes a resonant microcavity with an Er doped SiO2 active layer. This device emits radiation in the infrared region and is intended as a laser amplifier for fiber-optic communications.
The Schubert device, the semiconductor lasers and the LEDs are not as suitable for use in luminescent displays for several reasons. They contain luminescent materials such as Si, GaAs, etc., in the active region which are suitable as laser media, but which are typically inefficient emitters of visible light and require excitation by the injection of electrons. They also are designed with small planar surface areas that are inadequate for display purposes. Moreover, because of the design of these devices and the active materials used, they typically cannot be excited efficiently with electron bombardment, an electric field, or ultraviolet radiation. These excitation mechanisms are an essential part of the current display technologies.
Furthermore, the laser microcavity devices work above the laser threshold, with the result that their response is inherently nonlinear near this threshold and their brightness is limited to a narrow dynamic range. Displays, conversely, require a wide dynamic range of brightness. Microcavity lasers utilize stimulated emission and not spontaneous emission. As a result, these devices produce highly coherent light making these devices less suitable for use in displays. Highly coherent light exhibits a phenomenon called speckle. When viewed by the eye, highly coherent light appears as a pattern of alternating bright and dark regions of various sizes. To produce clear, images, luminescent displays must produce incoherent light.
In addition, it is important to distinguish the resonant microcavity display from the laser CRT. This display is similar to a CRT and scans an electron beam to write the information to the luminescent screen. However, the light is not produced by the spontaneous emission of the phosphor, but by stimulated emission. The faceplate of the laser CRT is an electron beam pumped semiconductor laser. The active medium, a semiconductor, is placed between two mirrors that form a laser cavity. The cavity structure is contained within the faceplate. When pumped with a sufficiently energetic electron beam, the device lases, producing a highly energetic and directional light beam. Such a display is described by A. S. Nasibov, et. al. in the article xe2x80x9cFull Color TV projector based on A2B6 electron-pumped semiconductor lasersxe2x80x9d, J. Crystal Gzowth, 117, 1040 (1992).
The subject invention, the Resonant Microcavity Display (RMD), is a luminescent display which offers the advantages of a thin-film phosphor without exhibiting the light piping problem. This is because it emits light in a highly directional manner as a result of its geometry.
The resonant microcavity display is any structure that modifies spontaneous emission properties of a phosphor contained within the structure. The modification of spontaneous emission is obtained by changing the optical mode amplitudes to the such a degree that the phosphor favorably emits into a relatively few optical modes. It is also possible to suppress emission in certain optical modes. This modification of mode amplitudes can be created, for example, by the formation of a standing wave electric field for each favored mode within the structure and locating the phosphor at the antinodes of these standing waves. It is essential that the standing waves have substantially modified electric field amplitudes relative to the the field amplitudes generated without a cavity. Substantially modified refers to changes by a factor of two or more in the field amplitudes.
In standing wave cavities, no enhancement can occur at the node of the electric field. However, a ring cavity design 320 such as that shown in the downward-looking view of FIG. 1 supports a traveling wave 322 in which the electric field amplitude is substantially modified throughout the entire cavity. As a result, mode enhancement or suppression can occur throughout the cavity. Compared to the standing wave cavity, more active medium 324 with modified light emission can be utilized for the same cavity volume.
One example of a resonant microcavity display is a microcavity resonator comprising a phosphor sandwiched between two reflectors, all of which are grown on a transparent rigid substrate. The width of the active region is chosen such that a resonant standing wave, of the wavelength to be emitted, is produced between the two reflectors. In its simplest form, a single coplanar microcavity, the two reflectors are parallel to each other and the plane of the active region is parallel to the reflectors. Other geometries which produce standing waves or traveling waves with an increased electric field amplitude, such as combinations of planar microcavities, three-dimensional microcavities, confocal microcavities, hemispherical microcavities, or ring cavities are also possible. These other geometries are well-known in the art of designing cavities.
Another structure that favorably alters the spontaneous emission properties uses photonic band gap crystals. A photonic band gap crystal can be formed from a monodispersed colloidal suspension. The structures comprise periodic dielectric media to create a band gap of energy for which light cannot propagate within the structure. However, doping such a structure with a material that has a resonance within the band gap will create a high Q cavity. Such cavities can be one, two or three dimensional. The cavity generates a standing wave with an enhanced electric field amplitude in the region of the dopant. In order to create a display, the photonic band gap crystals must be a phosphor. Henry O. Everitt describes photonic band gap crystals in xe2x80x9cApplications of Photonic Band Gap Structuresxe2x80x9d, Optics and Photonics News, 20, (1992). FIG. 2 is a side view of a resonant microcavity display 350 on a substrate 352 using a photonic band gap crystal 354 as the entire cavity structure.
Fabricating the RMD requires the use of a growth technique capable of controlling layer thickness or the spatial resolution of the refractive index to a precision of several nanometers. Such techniques, for example, include, but are not limited to, chemical vapor deposition (CVD), molecular beam epitaxy (MBE), atomic layer epitaxy (ALE), electron beam evaporation, or sputtering. Fabricating the RMD may also employ holographic photo-lithographic techniques. In this case, the Bragg reflectors are created by exposing a suitable material to a holographic pattern thereby creating in the material alternating layers of high and low refractive index regions. Such a technique is well known in the art of fabricating holographic diffraction gratings.
The substrate can be either a crystalline, polymer, or an amorphous solid. It can be made of any material that will allow the other regions to be grown on it. Suitable substrate materials may be chosen from a wide range of materials such as oxides, fluorides, aluminates, and silicates. The substrate material can also be fabricated using organic materials. The criteria involved in selecting a substrate material include its thermal conductivity and its compatibility (both physical and chemical) with other materials forming the RMD.
The phosphor may be excited through several means, including: bombardment by externally generated electrons (cathodoluminescence), excitation by electrodes placed across the active layer to create an electric field (electroluminescence), or excitation using photons (photoluminescence).
The present invention is distinguished from other microcavity devices in part by the placing of a phosphor in the resonant microcavity. Phosphors are materials that exhibit superior visible luminous efficiencies (where luminous efficiency, as used herein, is defined as the ratio of light output in Watts over the power input in Watts). Typically, the luminous efficiencies of phosphors range between 1% and 20%. These high efficiency materials are only classified as phosphors if the material efficiently generates luminescence when excited by electrons, electric fields, or light.
The active region may comprise a wide range of inorganic phosphors (e.g., sulfides, oxides, silicates, oxysulfides, and aluminates) most commonly activated with transition metals, rare earths or color centers. In addition to inorganic phosphors, the active region may employ an organic phosphor such as tris (8-hydroxyquinoline) aluminum complex. The active region comprises phosphors typically in the form of single crystal films, polycrystalline films, amorphous films, thin powder layers, liquids, or some combination of the above. A selection of phosphors that have found commercial applications, and from which an application dependent phosphor can typically be selected for use in the present invention, is documented in xe2x80x9cOptical Characteristics of Cathode Ray Tube Screens,xe2x80x9d Electronic Industries Association Publication TEP 116.
The reflectors forming the resonant cavity consist of either metallic layers or Bragg reflectors. Bragg reflectors are dielectric reflectors formed from alternating layers of materials with differing indices of refraction. The simplest geometry for dielectric reflectors consists of one-quarter wavelength thick layers of a low refractive index material, such as a fluoride or certain oxides, alternating with one-quarter wavelength thick layers of a high refractive index material, such as a sulfide, selenide, nitride, or certain oxides. The dielectric reflectors can also be fabricated using organic materials. Mirrors can also be formed using photonic band gap crystals. Any incident light with an energy within the band gap will be reflected by the structure. FIG. 3 shows a side view of an illustrative embodiment of a resonant microcavity display 340 on a substrate 342 in which an active layer 346 is sandwiched between two mirrors 344, 348 comprising photonic band gap crystals.
In current display applications, only one side of the screen is viewed. In the case of a microcavity, the design requires the use of different reflectors in order for most of the light to be projected towards the viewer. In the case of the simple coplanar microcavity, this asymmetry is obtained by having one of the two reflectors be substantially wholly reflective, meaning that it reflects most of the light impinging on it. The other reflector (opposite to the substantially wholly-reflective reflector) is partially reflective, meaning that it does not reflect as high of a percentage of impinging light as the wholly-reflective reflector and allows some of the light to pass through it. Because of the difference in reflectance of the two reflectors, virtually all of the light produced in the active region escapes through the partially-reflective reflector along the axis normal to the plane of the device.
In the case of a microcavity structure, the dimensions depend on the natural spontaneous emission spectrum of the phosphor being used, as observed outside of a cavity. If the spectrum covers a broad range of visible wavelengths it is possible to choose an appropriate part of the spectrum (i.e., one that matches an industry standard chromaticity) and construct the microcavity with a matching resonance. The final chromaticity of the RMD will correspond to the cavity resonance and will be different from the natural chromaticity of the phosphor outside of the microcavity. Conversely, if the phosphor""s natural spontaneous emission spectrum covers only a narrow range of visible wavelengths, the dimensions would be chosen so that the cavity resonance would match one of the phosphor""s emission bands.
The RMD has a highly directional light output similar to those of a projector or a flashlight and, as a result, RMDs can be constructed to avoid light piping. This allows highly efficient coupling to other devices. RMD""s also have a high external efficiency, approaching 100%. Since RMDs incorporate films, RMDs permit the design of efficient thermal conduction of the heat generated in the active layer. This feature combined with the ability to reduce the phosphor decay time allow RMDs to utilize intense excitation. As a result of the above, RMDs are especially suitable for use in projection displays.
It is therefore an object of this invention to provide a luminescent display that does not exhibit the problem of light piping.
It is a further object of this invention to provide a luminescent display with highly efficient heat transfer properties.
It is a further object of this invention to provide a luminescent display with a high external efficiency.
It is a further object of this invention to provide a luminescent display capable of high resolution.
It is a further object of this invention to provide a luminescent display which produces a highly directional output.
It is a further object of this invention to provide a luminescent display in which the chromaticity of the emitted light can be accurately controlled irrespective of the nature of the phosphor used.
It is a further object of this invention to provide a luminescent display wherein the phosphor used can be chosen to optimize the display with respect to properties other than chromaticity.
It is a further object of this invention to provide a luminescent display wherein the decay time of the activator can be tailored for the specific display application.
It is a further object of this invention to provide a luminescent display which can be heavily loaded by the excitation source without saturating the phosphor due to overheating.
Other objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description of the illustrated embodiments, when read in light of the accompanying drawings.
Througout this specification, published articles are cited for background purposes. These articles are hereby incorporated by reference into this specification.